(1) Field of the Invention
The invention relates to electrical test apparatus in general and more particularly to electrical test apparatus for testing the wiring of a ring type local area network (LAN) for short circuit, broken wire, terminal disconnect, etc.
(2) Prior Art
The use of LAN for transmitting data within a building or group of buildings is well known in the prior art. When transmission occurs between buildings, it is necessary that the buildings be positioned in close proximity. Usually the LAN consists of a wiring system to which a plurality of machines, hereinafter called Data Terminal Equipment (DTE), are connected.
The topology of the wiring system consists of a backbone or main ring formed from looped conductors and a plurality of interconnecting conductors tapped onto the main ring. The DTE's are coupled to the interconnecting conductors.
As with any type of wiring system, the aforementioned LAN is susceptible to wire fault. Such faults may be broken wires, unplugged terminals, short circuits, etc. The faults may occur during installation or develop after the wire is installed. In order to detect faults which may occur at installation and faults which may occur thereafter, the testing apparatus must be an integral part of the LAN. The testing apparatus must also be able to test for in situ fault.
The prior art discloses test devices suitable for testing multiconductor cable for continuity, correct connections, and the absence of short circuits between conductors. U.S. Pat. No. 3,982,180 is an example of such prior art. In the patent, each end of the cable to be tested is connected to terminating contacts. A structure consisting of a plurality of zener diodes is connected to one of the terminating contacts. The diodes have dissimilar zener breakdown voltages and are poled in the same sense in respect to a common junction point. The connection is such that a diode is coupled to one conductor in the cable. A test circuit is connected to the other terminating contact. The connection is such that at any instant a single conductor is placed in series with the other conductors collectively, a resistor, a current meter, and a DC voltage source. The voltage source is poled so as to cause the zener diode connected at the opposite end of the single conductor to have a zener breakdown when that conductor is continuously correctly connected and not short circuited to another conductor. The breakdown causes a unique reading of the meter. When the single conductor is non-continuous, incorrectly connected or shorted to another conductor, other meter indications are produced.
U.S. Pat. No. 3,983,338 is another example of the prior art testing devices. The testing device tests the operability of an electrical service line, such as a telephone service line, or other low voltage service line, between a test station and a telephone service installation or other electrical equipment connected to the line. An electrical device is connected to the line at the service installation. An electrical signal is transmitted from a test station to the electrical device. The electrical signal is selectively modified, by the electrical device, and returned to the test station. The modification is such that the test station can ascertain the operability of the line and/or the presence or removal of the telephone set or other electrical equipment connected to the line.
Although the prior art test devices work well for their intended purposes, they are not effective to test wire fault in an LAN. The ineffectiveness, in part, is due to the fact that the prior art test device cannot be integrated into the LAN.